1. Field of the Invention
The invention is generally related to the area of optical communications. In particular, the invention is related to improved designs of fiber optic interleavers based on retro-reflective structures.
2. The Background of Related Art
The future communication networks demand ever increasing bandwidths and flexibility to different communication protocols. DWDM (Dense Wavelength Division Multiplexing) is one of the key technologies for such optical fiber communication networks. DWDM employs multiple wavelengths in a single fiber to transmit in parallel different communication protocols and bit rates. Transmitting several channels in a single optical fiber at different wavelengths can multi-fold expand the transmission capacity of the existing optical transmission systems, and facilitating many functions in optical networking.
From a terminology""s viewpoint, a device that multiplexes different wavelength channels or groups of channels into one fiber is a multiplexer, and a device that divides the multiplexed channels or groups of channels into individual or subgroups of channels is a demultiplexer. Specifically, when a multiplexer combines two streams of spectrum-periodic optical signals into a single, denser spectrum-periodic signal stream, or in reverse a demultiplexer separates a single spectrum-periodic signal stream into two complementary spectrum-periodic signal streams, such multiplexer/demultiplexer is called an interleaver. Understandably, the spectrum-periodic signal stream is defined in the optical frequency domain and the periodicity is limited within certain optical fiber transmission band. The main function of an interleaver is to separate a channel-continuous spectrum-periodic signal stream into respective spectrum-periodic signal streams or vice versa.
A variety of technologies have been exploited to achieve an interleaver of high performance. Some of the technologies include fused fiber Mach Zehnder interferometer, multi-cavity Fabry Perot interferometer, and polarization interference filters consisting of birefringent crystals. These technologies generally suffer from poor crosstalk between signal channels. Crosstalk means any undesired signal leakage from one channel into another channel. The degradation caused by crosstalk is especially severe when a wide operating wavelength or wide operating temperature range is required, demanding additional active thermal control and resulting in reliability and other concerns. To minimize crosstalk, extremely high quality material and components with tight tolerances are needed. These materials and components increase the overall cost of the interleavers significantly.
Therefore, there is a need for an interleaver that can easily, economically, reliably and precisely demultiplex the desired channels from the multiple channels.
The present invention pertains to improved designs of fiber optical interleavers based on a retro-reflective structure. The retro-reflective structure includes three stages of optical components. A first stage including walk-off crystals, at least one of the crystals is used to displace a received optical signal with multiplexed bands or channels therein into a first ray (e.g. an O-ray) and a second ray (e.g. an E-ray). A second stage following the first stage includes at least a pair of birefringent crystals to form an interferometer that creates path or phase differences introduced by the birefringent index differences between the first ray and the second ray. A third stage following the second stage includes a pair of half wave plates, a walk-off crystal and a quarter wave plate, wherein the quarter wave plate has a highly reflective coating on one end to retro-reflect light beams back through the same stages. Respective reflected and processed first ray and second ray are then combined in the first stage to output respective demultiplexed beams.
An object of the invention is to provide an interleaver which can easily, economically, reliably and precisely separate desired channels from the multiple channels in an optical signal.
Other objects, features, and advantages of the present invention will become apparent upon examining the following detailed description of an embodiment thereof, taken in conjunction with the attached drawings.